1. Field of the Invention
This invention relates to a method for making a magnetic recording medium, on which information can be recorded at high densities. This invention particularly relates to a method for making a magnetic recording medium having a magnetic layer containing ferromagnetic grains, wherein surface properties of ferromagnetic grains are modified efficiently.
2. Description of the Prior Art
Nowadays there is a strong demand for magnetic recording media which exhibit magnetic recording characteristics suitable for the recording of information at high densities, for example, high coercive forces (Hc) and high residual magnetic flux densities (Br). As ferromagnetic grains to be contained in magnetic layers of such magnetic recording media, for example, ferromagnetic metal grains are suitable.
However, if the coercive force (Hc) and/or the residual magnetic flux density (Br) of ferromagnetic grains becomes high, the interaction between the ferromagnetic grains will also increase. Therefore, ferromagnetic grains, which exhibit a high coercive force (Hc) and/or a high residual magnetic flux density (Br), cannot easily be dispersed in magnetic coating compositions, which are used to form magnetic layers of magnetic recording media. Also, alloy grains are easily oxidized. Therefore, in cases where alloy grains are employed as the ferromagnetic grains in magnetic layers of magnetic recording media, the magnetic characteristics of the magnetic recording media more easily deteriorate with the passage of time than magnetic recording media, which have magnetic layers containing oxide types of ferromagnetic grains.
In order for better electromagnetic transducing characteristics to be obtained, ferromagnetic grains are used in the form of fine grains in the course of forming magnetic layers of magnetic recording media. Fine ferromagnetic grains have a higher level of interaction with one another than coarse ferromagnetic grains. Therefore, when fine ferromagnetic grains are employed, it becomes more difficult to achieve uniform dispersion of ferromagnetic grains.
In order for the aforesaid problems to be eliminated, it has been proposed to utilize various surface treating agents, such as surface active agents and reactive coupling agents, for the purposes of improving the dispersibility of ferromagnetic grains. Techniques for utilizing various surface active agents are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 52(1977)-156606, 53(1978)-15803, 53(1978)-116114, and 58(1983)-102504. Also, techniques for utilizing various reactive coupling agents are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 49(1974)-59608 and 56(1981)-58135 and Japanese Patent Publication No. 62(1987)-28489.
In the course of making a magnetic recording medium, such that a magnetic coating composition having good dispersing qualities may be obtained quickly, a process for kneading ferromagnetic grains and a small amount of a binder solution together has heretofore been carried out with an incorporating machine exhibiting a high shearing force, such as a kneader, before the ferromagnetic grains are dispersed in the binder solution by use of a dispersing machine, such as a ball mill or a sand grinder. Also, such that large kneading effects may be achieved and a uniform paste may be obtained from the kneading process, dispersing agents and/or stabilizers have heretofore been added to the mixture, which is to be kneaded, or binders have heretofore been added to the mixture, which is to be kneaded, in proportions falling within a specific range. Such techniques for making magnetic recording media are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 49(1974)-14537, 53(1978)-76012, 55(1980)-25406, and 58(1983)-200423.
As one of techniques for kneading a mixture under a shearing force higher than in conventional techniques, it has heretofore been known to minimize the amount of a solvent contained in the kneaded mixture. Specifically, with the known technique, such that a high shearing force may be obtained during the kneading process, it is necessary for the viscosity of the kneaded mixture to be kept as high as possible. However, in cases where the amount of the solvent contained in the kneaded mixture is kept as small as possible during the kneading process, it often occurs that the kneading process cannot be carried out appropriately or becomes impossible, depending on fluctuations in external factors, such as the temperature and humidity, and fluctuations in raw materials, particularly fluctuations in physical properties, for example, the water content, bulk density and specific surface area of the ferromagnetic grains.
It has also been proposed to treat ferromagnetic grains with dispersing agents and then to subject the treated ferromagnetic grains to kneading processes. Such techniques are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 53(1978)-76012 and 62(1987)-41274.
However, in order for ferromagnetic grains to be treated with dispersing agents or surface treating agents, filtered, and dried before being subjected to kneading processes, additional apparatuses must be used. Also, considerable time and expenses are required for the treating, filtering, and drying processes to be carried out. Additionally, solvents are used in the course of treating the surfaces of the ferromagnetic grains with dispersing agents. Therefore, the solvents come into contact with the ferromagnetic grains, and condensates, oxides, or the like, of the solvents are thereby caused to occur. The condensates, oxides, or the like, of the solvents are entrained into magnetic coating compositions, and consequently into magnetic layers constituted of the magnetic coating compositions. The condensates, oxides, or the like, of the solvents cause the magnetic layers to become plasticized and less durable. Moreover, the ferromagnetic grains come into contact with a small amount of oxygen in the course of being subjected to treatment, such as dipping in the solvents and drying of the solvents. Therefore, in cases where the ferromagnetic grains are ferromagnetic metal grains, problems also occur in that the magnetic properties of the ferromagnetic metal grains become poor during such treatment.
The inventors carried out research of how to treat ferromagnetic grains and found that, if ferromagnetic grains and a surface treating agent are mixed and ground without any solvent being used, processes can be simplified, dispersing qualities of the ferromagnetic grains in a magnetic layer can be kept good, and durability of the magnetic layer can be kept high. The present invention is based on such findings.